The invention relates to a dental handpiece comprising a drive shaft which is driven by a drive motor and which has two coaxial ring gears which can be selectively coupled to shafts passing through different tool carriers.
Handpieces of such type afford the advantage that tool carriers for tools involving a different speed of rotation only have to be connected to the appropriate ring gear in order to arrive at different drive transmission ratios for the tools.
In that respect, it is provided in EP-B1-0 012 871 that two coaxial ring gears are disposed on a common shaft and thus rotate at the same angular speed. In such arrangement the desired step-up or step-down transmission ratio is acheived by the tool carriers engaging by way of a small spur gear with the outer ring gear of the drive shaft or by way of a large spur gear engaging with the inner ring gear of the drive shaft. In that arrangement it is necessary for the shaft leading to the tool to be fitted to the drive shaft in an inclined and eccentric relationship, which on the one hand restricts the design options with regard to the shape of the handpiece while on the other hand requires the tool carrier to be of a relatively large diameter.
On the other hand, a construction of the type outlined in the opening part of this specification is known from EP-B1-0 012 872 (FIG. 10), in which a shaft can be arranged centrally in the tool carrier and various output drive speeds are achieved by using output drive shafts of different lengths. In that arrangement a gear at the end of the output drive shaft of the tool carrier that is towards the coupling means has a predetermined number of teeth, so that transmission ratios can only differ in accordance with differing numbers of teeth on the ring gears of the drive shaft. Another disadvantage with such construction is that the two transmission ratios require different directions of rotation.
It is already known, for example from U.S. Pat. No. 4,121,342, for different tool carriers to be driven selectively directly by way of a drive shaft or by way of a planetary transmission which is disposed around the drive shaft. The arrangement of a multi-stage planetary transmission (see DE-C-27 17 013) has also already been proposed, in order to be able to drive the output drive shaft of different tool carriers at different transmission ratios. A disadvantage of those handpieces which are necessarily straight in the region of the coupling means is the necessity of also having to effect manipulation in the handpiece for the purpose of changing the transmission ratio, in addition to exchanging the tool carrier.